In the past, prime movers which supply motive power to generating systems have been started from stand-still by either a dedicated starter motor or by components of the system which are subsequently driven by the prime mover to produce electrical power. The use of a dedicated starter motor, however, results in the addition of a little-used component to the generating system, thereby undesirably incresing the size and weight thereof. Thus, the latter approach of using the generating system components to start the prime mover has found increasing acceptance.
One example of the latter approach is disclosed in Mehl U.S. Pat. No. 4,481,459, assigned to the assignee of the present application. This patent discloses a brushless generator which is driven by a prime mover in a generating mode to produce AC power and which is capable of operation in a starting mode to develop motive power for starting the prime mover. In the starting mode, AC power at an increasing voltage and frequency is provided by an inverter to a permanent magnet generator (PMG) of the brushless generator. This accelerates a rotor common to the PMG and a main generator up to the synchronous speed of the main generator. Once this synchronous speed is reached, AC power at the synchronous frequency of the main generator is applied to the stator windings thereof to cause the main generator to operate as a synchronous motor and thereby develop starting torque. A torque converter which is coupled between the main generator and prime mover is then actuated to transfer the starting torque to the prime mover to bring it up to self sustaining speed.
Cronin U.S. Pat. No. 4,473,752, discloses a starter-generator machine which can be used to start a aircraft engine. The machine includes a rotor-shaped stator which is fixed within a squirrel cage induction rotor. The rotor in turn includes an array of magnets attached on the outer circumference thereof. The machine is operated as a starter by applying three-phase AC power to windings disposed within the rotor-shaped stator. This in turn accelerates the induction rotor and the permanent magnets. Once a predetermined rotor speed is reached, AC power is applied to stator windings in a outside stator surrounding the rotor to synchronize the rotating magnetic field developed by the permanent magnets with the rotating field created in the stator so that motive power is developed. The motive power is transferred to a prime mover to start same.
A starting system for starting a synchronous motor which in turns drives a generator is disclosed in Fletcher U.S. Pat. No. 3,867,677. An induction starting motor is provided AC power to accelerate the rotor of the synchronous motor and generator. Electrical power is thereafter supplied to the synchronous motor to cause it to develop motive power which is thereafter provided to the generator. By rotating the synchronous motor before applying power thereto, large power lines disturbances are said to be prevented.
Fletcher also discloses a circuit for operating the motor at unity power factor. A phase comparator compares the phase of the input voltage to the phase of the input current to develop an error signal which controls the motor field to keep the motor at unity power factor.
Steen U.S. Pat. No. 4,139,790, discloses a synchronous motor having a rotor which includes a plurality of magnets disposed in apertures within the rotor. A series of electrically conductive bars are disposed about the entire circumference of the rotor and the bars are connected together to form a squirrel cage winding. The Steen motor is operated in an induction mode by applying AC power to stator windings until the synchronous speed of the motor is substantially reached. Thereafter, synchronous operation is achieved due to the interaction of the magnet field developed by the magnets in the rotor with the magnetic field established by the current flowing in the stator windings.
Other patents disclosing permanent magnet rotors having in current conducting bars in a squirrel cage configuration include Morrill U.S. Pat. No. 2,432,436, Schaefer U.S. Pat. No. 2,488,437, and Brainard U.S. Pat. No. 2,493,102.
It has been found that the direct application of AC power to a generator to cause it to operate as a motor results in a large transient in the power provided to the generator. The magnitude of the transient may be such as to render portable supplies, such as ground power carts, unsuitable for this use. The attempt by the Fletcher et al. system to reduce power transients by rotating the synchronous generator rotor before applying power thereto is not entirely successful since both the synchronous motor and generator rotors are driven by the induction starting motor. This high-inertia load on the starting motor results in a significant transient at teh time AC power is applied to the induction motor.
Copending patent application Ser. No. 063,296, filed June 17, 1987 in the name of Raad et al, entitled "Prime Mover Starting System and Method" (Sundstrand Docket No. B02405-AT1-USA) discloses a starting system and method for starting a prime mover connected to a generator by a torque link. An induction motor is included having a torque rating substantially equal to the minimum torque required to rotate the rotor of the generator from rest to a particular speed in a predetermined time period while the torque link is deactuated. Also provided are a source of electrical power, contactors for connecting the source of electrical power to the induction motor when the torque link is deactuated to accelerate the generator rotor to the particular speed, a second set of contactors for connecting the source of electrical power to the generator windings once the generator rotor has reached a particular speed to cause the generator to operate as a synchronous motor and a torque link actuator for actuating the torque link once the generator is operating as synchronous motor to bring the prime mover up to starting speed. The usual large-magnitude transient developed in the source of electrical power is thus broken down into several transients of lesser magnitude.